Among the scuba diving devices now being generally used, there is an air tank to contain compressed air. There is also a buoyancy compensator jacket which may communicate with the air tank through an air hose. The air hose has a charge valve and a discharge valve for controlling air charging or discharging to adjust the buoyancy of the jacket.
An alarm device also often being included in the diving device. It may produce audio sound to alert an emergency situation and call for help. Conventional alarm devices use compressed air to repeatly produce pressure difference at two opposite sides of a membrane for generating vibration to produce audio sound to gain other people's attention.
However when a diver dives below water surface, sea water enters into the chamber and speaker of the alarm device making the vibration resulting from air pressure difference not functionable. Hence such alarm devices may only be used above water surface. It is useless below water surface.
There are many situations divers need help when diving under water surface such as physical exhaustion, body disorder, threatening by harmful seal animals or shark and the like. If a diver cannot get quick attention and help from other divers nearby, it could result in serious danger or even lose life. There is a great need for better alarm devices that can work effectively both above and under water surface.